Optical disk players used to play video disk, digital audio disk, etc., often experience difficulty following prerecorded tracks of information upon the disk, primarily when these tracks deviate from their standard path. To correct this tracking problem, disc players include a tracking servo apparatus, which includes a three-beam tracking system. More specifically, the three beam system includes a main central beam for reading prerecorded information and two tracking detector sub-beams arranged to precede and succeed the main beam, respectively, as the main beam moves perpendicular to the direction of disc rotation. Tracking sub-beams generate outputs A and B, which includes any disk information picked up by the sub-beams and represents their positions relative to the track intended to be read by the main beam. These three beams also lie along a line connecting the optical axes of each beam whereby the connecting line has a predetermined offset angle relative to a line tangential to the direction of disk rotation. A tracking error signal TE is generated from a difference (A-B) between the low frequency components, of outputs A and B generated by the tracking sub-beams. This error signal is then used to correctly position the main beam over a desired track.
Recently, however, technological advances allow denser information storage on each disk, which causes the pitch of recording tracks to becomes narrower. Hence, the conventional tracking error detector circuit is unable to distinguish between light and a dark (i.e., the change in the amplitude of an RF signal) when the sub-beams cross tracks. Consequently, an adequate tracking error signal cannot be generated and tracking servo integrity cannot be maintained. FIGS. 7(a)-7(c) show waveforms read from a disk in which the track pitch is large and FIGS. 8(a)-8(c) show waveforms from a disk in which the track pitch is small. FIGS. 7(a) and 8(a) show the main RF signal as the servo apparatus is moved radially across the disk. FIG. 7(b) and 8(b) show outputs A and B from tracking sub-beams moved in a similar radial motion, and FIG. 7(c) and 8(c) show respective error signals TE. It is apparent from FIG. 8(c) that small track pitch creates an error signal TE amplitude to small to correctly control the tracking servo apparatus.